Subsea cable having floodable optical fiber conduit

ABSTRACT

In at least some embodiments, a disclosed subsea cable includes one or more floodable optical fiber conduits each having at least one tight buffered optical fiber for transporting optical signals. Each tight buffered optical fiber may have a relatively limited length. The subsea cable may further include multiple strength members contra-helically wound around or together with the one or more floodable optical fiber conduits. There may also or alternatively be included at least one hermetically sealed optical fiber conduit having at least one protected optical fiber spliced to one of the tight buffered optical fibers. At least some implementations splice each of the tight buffered optical fibers to corresponding protected fibers for the long-haul communications. Flooding of the floodable conduits may be provided via connectors at the subsea cable ends, via breakout locations where sensors are attached, and/or via vents in the conduit wall. Some method embodiments deploy the disclosed subsea cable designs in a body of water, putting the interior of at least one floodable optical fiber conduit in fluid communication with the body of water while supporting extended use for communicating signals, particularly in deep water where temperatures are relatively low. Because the floodable conduits have pressure-equalized interiors they may be formed from plastic or other materials that ease the process of attaching sensors to the subsea cables.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Provisional U.S. Application Ser.No. 61/941,389, titled “Pressure-Balanced Subsea Optical Cable” andfiled Feb. 18, 2014 by Jeremy Crane Smith and Robert Alexis PeregrinFernighough, which is incorporated herein by reference.

BACKGROUND

Optical fibers are commonly employed for communicating data at highbandwidths. They have been viewed by the telecommunications industry asa crucial ingredient for evolving the communications infrastructure toits current state, and for that reason the investments to develop fiberoptic communications technology have focused on making such bandwidthsavailable over long distances. The long distances further necessitateproducing cables that are simultaneously affordable and robust.Telecommunications cables typically must resist not only the minortraumas associated with transport and installation, but also theinsidious effects of aging and long term exposure to the elements.

As one example, consider the well-known hydrogen darkening effect. Overa long exposure time, hydrogen, whether arising from corrosion,biological processes or other water-associated causes, migrates into thefiber core and reacts chemically with dopants or other impurities to“tint” the glass. Over short runs, the effect of this tint may be barelyperceptible, but over long distances the signal is overwhelmed by theresulting attenuation. Two other water-related degradation mechanismsare stress corrosion and zero-stress aging, both of which, over time,reduce the strength and transparency of unprotected optical fibers.

To combat these effects and improve communications cable robustness, theindustry has, through a great deal of investment and effort, developed astandard design approach. To protect optical fibers from hydrogendarkening and water-related degradation mechanisms, communicationscables route optical fibers through hermetically sealed stainless steeltubing. In the event of a pinhole or other flaw in the stainless steeltubing that might permit water to breach the barrier, gel or anotherfiller material in the tubing serve to prevent fluid migration along thelength of the tubing. Often, the stainless steel tubing is itself coatedto provide a redundant seal against water penetration, particularly inhigh pressure or marine applications.

Rather than reinventing the wheel, the geophysical surveying industryhas leveraged the technology developed by the telecommunicationsindustry for buried cables and subsea cables. Designs in development foroptical-communication-based survey streamers and seafloor cablestypically employ commercially available subsea telecommunications cablesand technology as the backbone of system designs. Despite themanufacturing difficulties and costs caused by the standard designapproach, these inherited precautions against exposing the opticalfibers to water represent the industry's accepted wisdom: common senseshared by technicians, engineers, and experts alike.

SUMMARY

In contravention of the accepted wisdom, it has been found that subseacables can successfully employ floodable optical fiber conduits for manyextended-exposure applications including permanent reservoir monitoring(PRM). This alternative cable design approach may be expected to yieldsubstantial savings in manufacturing costs. In at least someembodiments, a disclosed subsea cable includes one or more floodableoptical fiber conduits each having at least one tight buffered opticalfiber for transporting optical signals while in extended contact withwater. Each tight buffered optical fiber may have a length between 5 and200 meters, or in other embodiments, a length between 200 meters and 2km. The cable may further include multiple strength memberscontra-helically wound around or together with the one or more floodableoptical fiber conduits. There may also or alternatively be included atleast one hermetically sealed optical fiber conduit having at least oneprotected optical fiber that, in a completed cable, gets coupled to oneof the tight buffered optical fibers. At least some implementationssplice or otherwise connect each of the tight buffered optical fibers tocorresponding protected fibers for the long-haul communications.Flooding of the floodable conduits may be provided via connectors at thecable ends, via breakout locations where sensors are attached, and/orvia vents in the conduit wall.

Some method embodiments deploy the disclosed cable designs in a body ofwater, putting the interior of at least one floodable optical fiberconduit in fluid communication with the body of water. Due tolimitations on the length of the exposed fiber or other precautionsprovided herein, the cable can support extended use for communicatingsignals via the fibers in the flooded conduits, particularly in deepwater where temperatures are relatively low. To limit the length ofexposed fibers while at the same time providing for longer-distancecommunication capability, the exposed fibers may be spliced to protectedfibers in hermetically sealed conduits. Because the floodable conduitshave pressure-equalized interiors they may be formed from plastic orother materials that ease the process of attaching sensors to the cablesand that reduce the overall cost of raw cable.

BRIEF DESCRIPTION OF THE DRAWING

The drawing includes multiple figures:

FIG. 1 shows an illustrative hermetically sealed optical fiber conduit.

FIG. 2 shows an illustrative floodable optical fiber conduit.

FIG. 3 shows an illustrative galvanized steel wire.

FIG. 4 shows an illustrative subsea cable having floodable optical fiberconduits.

FIG. 5 shows an illustrative “splice can” connector.

FIG. 6 shows a simplified communication architecture schematic.

It should be understood, however, that the specific embodiments providedin the figures and detailed description below do not limit thedisclosure. On the contrary, they provide the foundation for one ofordinary skill to discern the alternative forms, equivalents, and othermodifications that are encompassed in the scope of the appended claims.

TERMINOLOGY

The terminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting. As used herein,the singular forms “a”, “an”, and “the” include singular and pluralreferents unless the content clearly dictates otherwise. Furthermore,the word “may” is used throughout this application in a permissive sense(i.e., having the potential to, being able to), not in a mandatory sense(i.e., must). The term “include,” and derivations thereof, mean“including, but not limited to.” The term “coupled” means directly orindirectly connected.

DETAILED DESCRIPTION

Fiber optic cables may be deployed in a number of underwaterenvironments, including subsea applications such as permanent reservoirmonitoring (PRM), where there is water exposure for relatively longperiods of time (e.g., months or years). In particular, PRM systems maybe designed for decades of operation in ultra-deep water (e.g., greaterthan 1500 m), while also remaining suitable for use at shallower depths.Conventional wisdom for such applications dictates the use of gel-filledstainless steel conduits for the optical fibers with robust hermeticseals at every connection and each of the sensor splices, whichtypically number in the hundreds and possibly the thousands. Each sealrepresents a cost, a time investment, and a potential failure point forthe system. Where the need for such seals (and associated costs andrisks) can be eliminated, manufacturing lead times can be reduced andmanufacturing efficiencies improved.

In this vein, it is noted that for short lengths of optical fiber (e.g.,on the scale of meters rather than kilometers), water exposure hastypically not been the root cause of failure. It is believed that thehydrogen darkening and water-related degradation mechanisms need not bea large concern (particularly when considered in a PRM environment) aslong as the length of exposed fiber is kept relatively short. Forexample, the exposed optical fiber length may in some instances be 1meter, 2 meters, 3 meters, 4 meters, 5 meters, 10 meters, 20 meters, 50meters, 100 meters, 200 meters, 300 meters, 400 meters, 500 meters, orany suitable range up to a maximum of about 2 kilometers or so(depending on the precise application).

Over such lengths, it is believed that hydrogen darkening would have arelatively negligible effect, even if it were to occur. But at the lowtemperatures typically encountered in most subsea applications(particularly those below 500 meters depth), hydrogen diffusion occursso slowly that no discernable darkening would be expected to occur overthe lifetime of a typical PRM application. Moreover, it appears that inthe anticipated operating environments for PRM, the low levels ofavailable hydrogen make such darkening even less of a concern.

In addition to limiting the lengths of the exposed fiber, certaindisclosed embodiments provide that the exposed fiber be adequatelycoated (“tight buffered”) with an appropriately chosen conformal polymerthat mitigates the effects of water molecule diffusion. In unprotectedoptical fibers, water molecules may expand pre-existing surface flaws,causing crack propagation in a fashion similar to water freezing andexpanding a crack within a cement surface. If this coating (buffer)resists delamination or otherwise remains adhered to the optical fiber,it may prevent the water molecule diffusion process from leading tofailures. In some instances, it may achieve this in the following way:as water diffuses through the polymeric buffer, a silicate layer mayform on the surface of the glass, but will typically not migrate,instead remaining conformal with the buffer and blocking furtherdiffusion of other water molecules. Thus the buffer essentially haltsthe mechanical degradation caused by crack growth.

Previous PRM cable designs attached sensors (e.g., accelerometers,hydrophones, etc.) to the PRM cable at regular distances (e.g., every 50meters in some instances). The optical fibers in the outer cable layerwere accessed (broken out of the cable) at these locations forattachment to the sensors. These access points were then pressure-sealedto prevent the ingress of water. In contrast, the PRM cable designsdisclosed herein may be specifically configured to use the locations ofeach of the attached sensors in the sensor array as points of entry forthe water to flood the optical fiber conduits between sensor locations.These conduits may in some embodiments be either an engineered polymericmaterial or corrosion-resistant steel. They may also be considered“dry,” in the sense that they need not be filled with gel as hastypically been done in subsea cables. With this approach, the conduitsmay become pressure-balanced (no differential pressure across the tubewall) in the subsea environment. This allows for a much wider selectionof lower cost materials and processing methods, as well as removing thenecessity for high-pressure seals throughout the system.

To enable exposed optical fiber lengths to be limited in long cables,some disclosed cable embodiments also incorporate the traditionalhermetically sealed, gel-filled tubes within an inner layer in thecable. At the end of each cable section (feasible cable section lengthsmay be in the range of 500 m to 5 km), these hermetically sealedconduits may be accessed to connect selected optical fibers to exposedoptical fibers in the outer cable layer and to connect the rest of theinner conduit fibers to the hermetically sealed conduits in adjacentsections. Space to connect the sections and protect the splices isprovided by pressure sealed modules (referred to as “splice cans”) whichare typically maintained at 1 atmosphere while deployed subsea. Thus atleast some cable embodiments have strings of hermetically sealed opticalfiber conduits that may extend the full length of the cable. Therefore,these inner fibers need not be exposed to the outside environment.Within the pressure sealed modules, the outer flooded fibers may bespliced (or otherwise connected) to the inner sealed fibers insubsequent sections, thus limiting the exposed fiber to a manageablelength. In addition to splicing, suitable connection methods includecoupling via passive splitters, amplifiers, and active multiplexers ofany kind Active multiplexers may include amplifiers, filters, switches,frequency shifters, demodulators, buffers, protocol converters, andmodulators.

Turning now to the figures, FIG. 1 shows an illustrative hermeticallysealed optical fiber conduit 100. The illustrative hermetically sealedoptical fiber conduit 100 carries twenty optical fibers 102 within agel-filled interior 104 of a stainless steel through tube 106 having anexternal waterproof layer 108. In other words, the optical fibers 102are protected optical fibers. (Different embodiment may have greater orlesser numbers of optical fibers 102.) In at least some embodiments, theoptical fibers 102 are each single-mode, low water peak, 250 microndiameter, dual acrylate optical fibers compliant with InternationalTelecommunications Union (ITU) standard ITU-T G.652.D. In at least someembodiments, the water-blocking gel fills at least 85% of the remainingvolume of the tubing interior 104. The gel may include carbon or otherdopants to capture available hydrogen before it diffuses into the fiber.In at least some embodiments, the through tube 106 consists of 316Lstainless steel with an outer diameter of about 2 mm and a wallthickness on the order of 50 microns. The waterproof layer 108 providesa redundant seal against imperfections in the tube 106, and may be asheath of a high density polyethylene (HDPE) with a PolyBond™ additive(or other compatibilizing agent that lowers the interfacial surfaceenergy to promote bonding with the metal tube) and an optional colorant,giving the hermetically sealed optical fiber conduit 100 a total outerdiameter of approximately 3.0 mm. The optical fibers 102 may be providedwith a minimum of 0.3% excess length relative to the length of the tube106 to accommodate differing strains of the various conduit materials.

FIG. 2 shows an illustrative floodable optical fiber conduit 200. Theillustrative conduit floodable optical fiber 200 carries four opticalfibers 202 within a “dry” (i.e., not gel-filled) interior 204 of a looseplastic tube 206 potentially having periodic vents 207 to provide fluidcommunication across the tubing wall. (The vents 207 are optional, asflooding or fluid communication between the exterior and the interior204 may alternatively or additionally be enabled from the ends of thefloodable optical fiber conduit 200 and anywhere breaks are made toaccess the optical fibers 202.) The optical fibers 202 are each providedwith a tight buffer layer 203. That is, the buffer layers 203 conformwith the outer surface of the optical fibers 202 and adhere in a fashionthat resists separation from the outer surface in the presence of water.In other words, the optical fibers 202 are tight buffered opticalfibers.

In at least some embodiments, the optical fibers 202 are eachsingle-mode, low water peak optical fibers compliant with InternationalTelecommunications Union (ITU) standard ITU-T G.657.A1, having anacrylate outer diameter of 250 microns and a buffer layer outer diameterof 500 to 900 microns. The tight buffer layer 203 may be polymeric,using a thermoplastic elastomer such as Hytrel® from DuPont or athermoplastic fluoropolymer such as Kynar® polyvinylidene fluoride(PVDF) from Arkema, both of which offer tight conformal coatings thatresist delamination. Though both materials are extremely stable inwater, the latter offers a particularly low hydrogen permeability.Colorants may be added to the buffer material to make the optical fibersreadily distinguishable.

In at least some embodiments, the loose plastic tube 206 consists ofpolypropylene or PVDF with an outer diameter of 3.0 mm and an innerdiameter of 2.0 mm. Colorants may be included to make multiple suchfloodable optical fiber conduits 200 readily distinguishable. Theoptical fibers 202 may be provided with a minimum of 0.3% excess lengthrelative to the length of the loose plastic tube to accommodatediffering strains of the various conduit materials.

FIG. 3 shows an illustrative galvanized steel wire 300. In at least someembodiments, the galvanized steel wire 300 consists of high-strengthsteel with an outer diameter of about 3.2 mm. The galvanized steel wire300 is galvanized for corrosion resistance or may alternatively beprovided with a Galfan coating. It is noted that the galvanized steelwire functions mainly as a long-lived flexible strength member and forsome applications may be replaced with other wire or strand materialsthat provide adequate design strengths. Such materials may include othermetals, polymers, and natural fibers.

FIG. 4 shows an illustrative subsea cable 400 having one or morefloodable optical fiber conduits. The illustrative subsea cable 400 hasthree strand layers, with a center layer consisting of a galvanizedsteel wire 300A. Wound helically around the center layer is a secondlayer of six strands. The second layer has three galvanized steel wires300B interspersed with three hermetically sealed optical fiber conduits100. Helically contra-wound around the second layer is an outer layer oftwelve strands. (The relative winding pitches may be chosen to providetorque balancing between the layers.) Two out of every three outerstrands are galvanized steel wires 300C, and the third of every threeouter strands is a floodable optical fiber conduit 200. Alternativeembodiments include other combinations and configurations ofhermetically sealed optical fiber conduits 100, floodable optical fiberconduits 200, and galvanized steel wire 300 within subsea cable 400.Enclosing the strands is an outer cable jacket 404, potentially havingperiodic vents 406 to provide fluid communication between the exteriorand interior of the cable. (The vents 406 are optional, as flooding orfluid communication between the exterior and the interior mayalternatively or additionally be enabled from the ends of the subseacable 400 and anywhere breaks are made to access the floodable opticalfiber conduits 200. Vents 406 may be of any size or shape capable ofproviding flooding or fluid communication between the exterior and theinterior of outer cable jacket 404.) A polymeric bedding layer 402 mayenclose the outer strand layer, extending into the interstices betweenthe outer layer strands to provide additional cable crush resistance andminimize the flex-induced bearing forces exerted by the galvanized steelwires on the floodable conduits. If vents 406 are provided in the outercable jacket 404, the vents 406 may also penetrate the polymeric beddinglayer 402. In at least some embodiments, outer cable jacket 404 consistsof a high density polyethylene (HDPE) material.

FIG. 5 shows a partially-exploded view of an illustrative connector ofthe “splice can” variety for coupling together different segments ofsubsea cable 400. A connector cone 502 is placed over the end of eachsegment and secured to the subsea cable 400, preferably by mechanicalattachment to the galvanized steel wires 300. The connector cones 502provide room to spread and route the optical fibers from thehermetically sealed conduits 100 and the floodable conduits 200. Theoptical fibers may be routed via feedthroughs in pressure plates 504,which feedthroughs are then sealed before being mounted to the connectorcones 502. On the pressure-controlled side of the pressure plates 504,the optical fibers are labeled 508. Frame rails 506 attach the pressureplates 504 together and support a connection module assembly 510 thatinterconnects the appropriate optical fibers 508. As previouslymentioned, such interconnections may be performed using splicing,passive couplers, amplifiers, and/or active multiplexers.

A housing 514 attaches to the pressure plates 504, forming ahermetically sealed canister that shields the optical fibers 508 andconnection module assembly 510 from exposure to pressure or water. Theconnector cones 502 are not sealed, and consequently they permitflooding of interiors. Vents 518 may be provided to facilitate suchflooding. The floodable conduits 200 are connected to the space insidethe connector cones 502 so that the interiors are fluidly coupled to theexterior of the subsea cable 400 via these spaces.

FIG. 6 shows a simplified communication architecture schematic, toillustrate how the interconnection of optical fibers from thehermetically sealed conduits and the floodable conduits might beperformed. FIG. 6 shows a segment 602 of subsea cable 400 coupled byconnectors 604 to its neighboring segments 606. Spaced along the lengthof segment 602 (and neighboring segments 606) are geophysical surveyenergy sensors 608. Each of these sensors 608 is coupled (as representedby dots 610) to an associated fiber 612 in a floodable conduit. At theconnectors 604, each of the floodable conduit fibers 612 is coupled (asrepresented by dots 614) to a fiber in a hermetically sealed conduit616. The connectors 604 further provide coupling between correspondingfibers in the hermetically sealed conduit of segment 602 and itsneighboring segments 606.

The segmented subsea cables may be assembled with sensors of geophysicalenergy (e.g., seismic, electromagnetic) and deployed in a body of wateras spatially-distributed on-bottom cables. The deployed cables have atleast one floodable optical fiber conduit with an interior in fluidcommunication with the body of water. The cables are coupled at one endto an interface that supplies optical interrogation signals to theoptical fibers in the backbone, i.e., in the hermetically sealedconduits. The connection module assemblies cooperatively distribute theinterrogation signals to the sensors and return the modulatedmeasurement signals from the sensors to the interface, where they may becaptured in digital form by a measurement data acquisition system andstored for later processing.

Between the connection module assemblies and the sensors, the opticalsignals are each communicated via at least one tight buffered opticalfiber along the floodable optical fiber conduit, and between theinterface and the connection module assemblies, the optical signals areeach transported over at least one protected optical fiber in ahermetically sealed optical fiber conduit. The tight buffered opticalfiber has a length of no more than 2 km, though lengths as short as 5 mare also contemplated, and may have a thermoplastic elastomer or athermoplastic fluoropolymer as a conformal buffer layer material.

The on-bottom cables may be deployed in water bodies having depths inexcess of 500 m, and are expected to by employed periodically over aperiod of at least ten years for, e.g., PRM operations. PRM operationsmay involve seismic shots to generate seismic waves or controlled sourceelectromagnetic (CSEM) transmissions to generate electromagnetic fields,or such operations may be entirely passive, monitoring sensor responsesto background sources of such waves and fields. The sensor signals areprocessed to map subsurface distributions of fluid and rock propertiesand to track changes to such distributions over time, e.g., ashydrocarbons are produced from subsurface reservoirs.

The processing may further produce geophysical data in intermediateprocessing stages such as re-sampled and stacked traces, migrated andfiltered data volumes, and imaged subvolumes. Each of these, includingthe raw acquired data and the final maps of subsurface propertydistributions, can be packaged as a geophysical data product andrecorded on a tangible, nonvolatile computer readable storage mediumsuitable for importing the geophysical data product onshore. Suchgeophysical data products can be further subjected to geophysicalanalysis and interpretation, e.g., to develop and optimize reservoirproduction strategies.

In accordance with an embodiment of the invention, a geophysical dataproduct may be produced. The geophysical data product may includeseismic data communicated as signals via a tight buffered optical fiberalong a floodable optical fiber conduit which may be stored on anon-transitory, tangible computer-readable medium. The geophysical dataproduct may be produced offshore (i.e. by equipment on a vessel) oronshore (i.e. at a facility on land) either within the United States orin another country. If the geophysical data product is produced offshoreor in another country, it may be imported onshore to a facility in theUnited States. Once onshore in the United States, geophysical analysis,including further data processing, may be performed on the geophysicaldata product.

Though the foregoing description presents specific cable embodimentswhich may be used to satisfy various challenges presented by the extremeenvironments presented by PRM installation and design life, suchembodiments may also provide a solution that is applicable acrossmultiple other scenarios and applications. Moreover, there are otherunderwater and underground cable embodiments within the scope of thisdisclosure having greater or fewer numbers of floodable optical fiberconduits with greater or lesser numbers of tight buffered opticalfibers. Where long length cables are desired, such cables may furtherinclude hermetically sealed conduits with protected optical fibers thatcouple to optical fibers of the floodable conduits in a manner thatlimits the lengths of the exposed fibers. As required by other designconstraints, additional strength members may be included along withprotective jackets. The rated lifetimes for the disclosed cables may beexpected to exceed 10 years, 20 years, 25 years, or more.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure. Thescope of the present disclosure includes any feature or combination offeatures disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Various advantages of the present disclosurehave been described herein, but embodiments may provide some, all, ornone of such advantages, or may provide other advantages.

What is claimed is:
 1. A subsea cable that comprises: one or morefloodable optical fiber conduits each having at least one tight bufferedoptical fiber for transporting optical signals, wherein each tightbuffered optical fiber comprises: a plastic-coated optical fiber; and athermoplastic elastomer or a thermoplastic fluoropolymer as a conformalcoating on the plastic-coated optical fiber.
 2. The subsea cable ofclaim 1, wherein each tight buffered optical fiber has a length between5 m and 2 km.
 3. The subsea cable of claim 1, further comprisingmultiple strength members helically wound around or together with theone or more floodable optical fiber conduits.
 4. The subsea cable ofclaim 1, further comprising at least one hermetically sealed opticalfiber conduit having at least one protected optical fiber spliced to atight buffered optical fiber from the one or more floodable opticalfiber conduits.
 5. The subsea cable of claim 4, wherein each of multipletight buffered optical fibers from the one or more floodable opticalfiber conduits is spliced to a corresponding protected optical fiber inthe at least one hermetically sealed optical fiber conduit.
 6. Thesubsea cable of claim 5, further comprising: two cable segments; and aconnector to join an end of one cable segment to an end of the othercable segment, wherein splices between the tight buffered optical fibersand the protected optical fibers are made within the connector.
 7. Thesubsea cable of claim 6, wherein interiors of the one or more floodableoptical fiber conduits are in fluid communication with an exterior ofthe subsea cable via the connector.
 8. The subsea cable of claim 1,wherein interiors of the one or more floodable optical fiber conduitsare in fluid communication with an exterior of the subsea cable via oneor more vents in a wall of the floodable optical fiber conduit.
 9. Thesubsea cable of claim 1, further comprising an array of sensors attachedalong a length of the subsea cable, each sensor in the array coupled toat least one tight buffered optical fiber from a floodable optical fiberconduit and providing fluid communication between an interior of thefloodable optical fiber conduit and an exterior of the subsea cable. 10.The subsea cable of claim 1, wherein the conformal coating is configuredto at least one of: mitigate effects of water molecule diffusion, resistseparation from an outer surface of the plastic-coated optical fiber,resist delamination, be extremely stable in water, and have a lowhydrogen permeability.
 11. The subsea cable of claim 1, wherein theplastic-coated optical fiber is at least one of: a single-mode opticalfiber, a low water peak optical fiber, a 250 micron diameter opticalfiber, an acrylate-coated optical fiber, and a dual-acrylate opticalfiber.
 12. An apparatus, comprising: a tube having a length between 1and 100 meters; and a plastic-coated optical fiber disposed within thetube and having a polymeric buffer conformally disposed on an outersurface thereof; wherein, when the apparatus is submerged in a fluid,the tube is configured to allow the fluid to contact the polymericbuffer and to equalize an interior pressure of the tube and an exteriorpressure of the tube.
 13. The apparatus of claim 12, wherein thepolymeric buffer is a thermoplastic elastomer or a thermoplasticfluoropolymer.
 14. The apparatus of claim 12, wherein the apparatusfurther comprises a hermetically sealed optical fiber conduit having atleast one protected optical fiber disposed therein to transport signalsfrom the polymeric-buffered plastic-coated optical fiber over a distancethat exceeds the length of the tube.
 15. A subsea cable that comprises:one or more floodable optical fiber conduits each having at least onetight buffered optical fiber for transporting optical signals, whereineach tight buffered optical fiber has a thermoplastic elastomer or athermoplastic fluoropolymer as a conformal buffer layer material; atleast one hermetically sealed optical fiber conduit having at least oneprotected optical fiber spliced to a tight buffered optical fiber fromthe one or more floodable optical fiber conduits, wherein each ofmultiple tight buffered optical fibers from the one or more floodableoptical fiber conduits is spliced to a corresponding protected opticalfiber in the at least one hermetically sealed optical fiber conduit; twocable segments; and a connector to join an end of one cable segment toan end of the other cable segment, wherein splices between the tightbuffered optical fibers and the protected optical fibers are made withinthe connector, wherein interiors of the one or more floodable opticalfiber conduits are in fluid communication with an exterior of the subseacable via the connector.
 16. The subsea cable of claim 15, wherein eachtight buffered optical fiber has a length between 5 m and 2 km.
 17. Thesubsea cable of claim 15, further comprising multiple strength membershelically wound around or together with the one or more floodableoptical fiber conduits.
 18. The subsea cable of claim 15, whereininteriors of the one or more floodable optical fiber conduits are influid communication with an exterior of the subsea cable via one or morevents in a wall of the floodable optical fiber conduit.
 19. The subseacable of claim 15, further comprising an array of sensors attached alonga length of the subsea cable, each sensor in the array coupled to atleast one tight buffered optical fiber from a floodable optical fiberconduit and providing fluid communication between an interior of thefloodable optical fiber conduit and an exterior of the subsea cable.